The present invention relates to a method for measuring antigen concentration in a sample. It relates more specifically to a new method for making more simple and highly sensitive measurements of antigen concentration by employing intramolecular interactions between components of antibody (immunoglobulin). This method can be utilized as, for example, a sandwich ELISA method, and is useful for clinical diagnosis of the concentrations of various antigens in biological samples (blood, urine and body fluid). It is also applicable to the specific detection of all other sorts of substances, both natural and unnatural, recognizable as an antigen, and can be used as research and industrial sensors.